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Message-ID: <3DC51F30.A20227E0@webaccess.net>
From: Chuck Simmons
Organization: You jest.
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Newsgroups: sci.electronics.design
Subject: Re: Constant Frequency at LOW temperature
References:
Date: Sun, 03 Nov 2002 13:06:11 GMT
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Arch-lab wrote:
>
> Feq. isn't pinned down yet since my client hasn't told me yet what the gear
> ratios are that they are building. However, the application will use a
> stepper. As far as accuracy, basically it will have to be able to keep up
> with the sky (telescope tracking).
Telescope tracking is a pretty complex problem. For a telescope to have
reasonable basic tracking the stability has to be good but the base
frequency is not very important because of all of the corrections
needed. If the master oscillator is good to 1 second of time per day (15
arcseconds per day), it has a stability over the period equivalent to an
accumulated error of about 0.001%. Although this is nicely in the realm
of reason for a quartz oscillator, clearly the master oscillator must be
referred to a reference several times per day (LF radio is usual). That
can reduce accumulated error to less than 200 milliarcseconds per day
with the benefit that long term accumulated error goes to zero.
The tracking problem is complex because the tracking device must
characterize and track out mechanical errors such as periodic worm error
and the like. Due to limited rotation, polynomial fits to mechanical
errors are common. Another error source is aiming of the polar axis for
equatorial mounts and errors in finding celestial north for el-az
mounts. These introduce rate errors. Refraction is also an important
error source and refraction must be taken into account in the drive
rate.
Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons chrlsim@webaccess.net